The invention relates generally to the field of rotating machines, and in particular to turbine engines. Specifically, embodiments of the present technique provide a compliant seal between rotating and static components in such machines.
A number of applications call for sealing arrangements between rotating and stationary components. Such seals may vary in construction, depending upon such factors as the environments in which they function, the fluids against which they form a seal, and the temperature ranges in which they are anticipated to operate. In turbine and similar applications, for example, seals are generally provided between the various stages of rotating components, such as turbine blades, and corresponding stationary structures, such as housings or shrouds within which the rotating components turn.
Efficiency and performance of gas and steam turbines are affected by clearances between rotating blade tips and the stationary shrouds, as well as between the nozzle tips and the rotor. In the design of gas and steam turbines, it is desirable to have a close tolerance between the tips of the rotating blades and the surrounding static shroud. In a turbine engine, the portion of the working fluid passing through the clearance between the tips of the rotating blades and the stationary shroud does no work on the blades, and leads to a reduced efficiency of the engine. Generally, the closer the shroud or stationary component surrounds the tips of the rotating blades, the greater is the efficiency of the turbine engine.
However, clearance dimensions between the rotating blade tips and the stationary shroud may vary at different times during the operation of the turbine engine. For example, the clearance decreases significantly due to dissimilar thermal growths, non-uniformity or transient motion between adjacent rotating and static components, causing interfacing surfaces to rub. Such a rub may lead to rapid wear of the blade and the stationary shroud, and may set up forced vibrations in the turbine engine. Wear on the shroud and the rotating blades is undesirable as it increases clearance dimensions and leads to a further loss in efficiency.
Prior methods to solve the above problem include using a seal on the stationary shroud surface, the sealing material being designed to be wearable or abradable with respect to the rotating blade rubbing against them. In such a system, a rub or contact of the blade tips with the stationary shroud causes the abradable shroud material to abrade or flake off. This avoids damage to the rotating components, and provides reduced clearances and thus better sealing as compared to a non-abradable system, in which large cold-built clearances have to be provided to prevent rubbing during transient conditions, such as dissimilar thermal growths between rotating and static components. However, this abradable system suffers from the disadvantage of reduced life of the sealing material. Also, previous abradable seals, even though various materials for the shroud have been proposed such as sintered metal, metal honeycombs and porous ceramics, have not provided a desirable compliance. Further, after a rub or a contact due to a transient condition, the gap or wear produced by the rub or contact is larger than the interference depth, due to tearing out, galling and spalling.
Accordingly, there is a need for a sealing technique to minimize the damage caused to the rotating and static components due to rubbing during transient periods, and to reduce vibration levels in the turbine engine caused by the same.